You're not going to find the answer in writing anywhere. I'll spell out the two sides of the argument.
1. Yes you can do it. The ATF (in the Thompson Center ruling, and things like the FAQ above) has said you can go from pistol -> rifle -> pistol as long as you don't configure it as an illegal SBR in the process. SBRs are a subset of rifles, so therefore if pistol -> rifle -> pistol is legal, then it stands to reason that pistol -> SBR -> pistol is legal. The assumption is the firearm starts life as a pistol.
2. No you can't do it. When you file a Form 1, you are making a new firearm...hence the title of the form (Application to Make and Register...). And that new firearm, by definition, is a Short Barreled Rifle. Doesn't matter if you're starting with an 80% lower or an existing firearm and adopting the serial number. You're still making a new firearm (and engraving it as maker). So, since it starts as a rifle, it can't be converted to a pistol. If you remove the stock, you don't have a pistol...you have a weapon made from a rifle, which is an SBR.
So, that's the two sides. I see the logic in both. The "safe" answer is #2...but that doesn't make it the right answer, it just guarantees you're not in the wrong.
I've seen one email response from Gary S @ NFA Division that said it was OK. But that's all the "put it in writing" I've ever found.